I just found this on Pinterest pinned by a girl who said she graduated high school without a grade lower than A. I cannot confirm that the list is accurate, but here are the websites:
TextBookNova.com
eBookee.org
ManyBooks.net
FeedUrBrain.com
AllenG.ru (with the "ru" ending, I am assuming it is from Russia)
2020Ok.com
FreeTextBooks.com
Gutenberg.org
Eknigu.com
En.Bookfi.org